Analysis of many different types of data collected throughout the world
has shown that climate can change abruptly. The record is roughly 6°C
in 1–3 years recorded in Greenland ice cores (Steffensen et al
2008) and in a core through sediments in a German lake (Brauer et al
2008). Ice cores from Antarctica and Greenland show:

Rapid climate changes, between multiple nearly stable states, can
and do occur. Look at the plot of temperature on
the Carbon Dioxide Problem web page.
Notice the different periods of relative constant temperature:

0 to 12,000 years ago at a temperature in Greenland
of 0°C.

15,000 to 30,000 years
ago at a temperature in Greenland of –8°C.

90,000
and 120,000 years
ago at a temperature in Greenland of –4°C.

200,000 to 210,000 years
ago at a temperature in Greenland of –2°C.

Thus we
see five periods of nearly stable climate lasting tens of thousands
of years. Many others are seen in the plot, lasting hundreds
to thousands of years.
These are examples of multiple nearly
stable climate states separated by times of rapid (abrupt) change.

An Example
Here is an example from the period called The Younger Dryas seen
clearly in European Greenland data. This period is named after
a flower (Dryas octopetala) that grows in cold conditions and that
became common in Europe during this time.Central Greenland temperature over the past 20,000 years from analysis of oxygen
isotope ratios in Greenland ice cores.
From NOAA The
Younger Dryas cold interval as viewed from central Greenland.

Transitions in deuterium isotope concentration (d) observed
in Greenland ice cores sampled with a resolution of 1–3 years. The deuterium
isotopes are closely related to temperature of the oceanic area that supplied
moisture that led to Greenland precipitation. The data show that the atmosphere
switched mode within 1 to 3 years (horizontal grey bands) during these transitions
and initiated a more gradual change (over 50 years) of the Greenland air temperature,
as recorded by stable water isotopes.
From Steffensen et al 2008).

Abrupt changes in the deep mass circulation (indicated in part by
comparison of Antarctic and Greenland ice core records) have taken
place, associated with large and rapid changes in climate.

There are strong linkages between the physical and biogeochemical
aspects of the earth system. They show up in variations of key atmospheric
trace constituents such as CO2, CH4, N2O
and aerosols (small particles in the air, not the gas in an aerosol
can).

The earth system is characterized by both positive (self-reinforcing)
feedbacks which lead to rapid changes and negative (self-limiting)
feedbacks which lead to more-or-less stable periods of CO2 concentration
in the ice cores.

From Newsletter of the Global, Analysis, Integration,
and Modeling (GAIM) Taskforce of the International Geosphere Biosphere
Program, Summer
2003, page 13-14.

The possibility of abrupt climate change due to changes in the deep
circulation of the north Atlantic was first proposed by Wallace
Broecker of the Lamont-Doherty Observatory of Columbia University.
He is the one who wrote:

"... it is clear that Earth's climate
system has proven itself to be an angry beast. When nudged, it is capable
of a violent response." From: Broecker
(2003).

The ocean and atmosphere transport heat from the tropics
to high latitudes. The ocean transport is important at the lower latitudes,
and the atmosphere is important at higher latitudes. See the zonal
average of heat transport from the Ocean
and Climate web page. The meridional overturning circulation in the
Atlantic plays an important role in the oceanic heat transport system
and abrupt climate change.

The Meridional Overturning Circulation in the Atlantic
In winter,
the surface waters of the North Atlantic Ocean cool and become so cold
and dense that they sink to the bottom. The sinking water carries CO2 deep
into the ocean, removing it from contact with the atmosphere. The sinking
water must be replaced by northward transport of water at the
surface. Because the Atlantic is connected to the rest of the ocean through
the Antarctic Circumpolar Current, the replacement water must come all
the way from the far South Atlantic.

The surface (red) and deep (blue) circulation
in the North Atlantic. The sinking of water in the Norwegian and Greenland
Seas helps keep the far North Atlantic ice free, leading to a warmer
Europe. From: Woods Hole Oceanographic Institution.

As the water moves northward along
the surface, it passes the tropics where it is warmed to nearly 28° C.
Thus, heat transport in the Atlantic is everywhere to the north, even
in the southern hemisphere. All the solar heat absorbed by the Atlantic,
about one petawatt, is carried northward to warm the northern hemisphere.
The heat is lost when the warm water cools keeps the far North Atlantic
ice free, and it helps keep Europe warm.

Northward heat transport for 1988 in each ocean
and the total transport summed over all oceans.
From Houghton et al., (1996: 212), which used data from Trenberth and Solomon
(1994).

Abrupt Changes in the Meridional Overturning Circulation
Ice and ocean sediment core data show many instances of the northern-hemisphere
climate changing abruptly. The change seems to be related to changes
in the meridional overturning circulation. Here is what
happened:

Large numbers of icebergs were dumped into the North Atlantic by
continental glaciers. The icebergs carried sand and gravel which fell
to the bottom as the ice melted. The layers of sand and gravel are
seen in sediment cores (numbered 2 and 3 in the figure below).

Ice cores on Greenland (number 1 in the figure below), show abrupt
cooling. The cold period lasts about a thousand years, then the air
over Greenland warms.

During the cold periods, the polar front shifts southward. The front
separates relatively warm surface water in the Atlantic from ice-covered,
cold polar water. Ice covered water extends almost as far south as
the Mediterranean Sea.

Periodic surges of icebergs during the last
ice age appear to have modulated temperatures of the northern hemisphere
by lowering the salinity of the far north Atlantic and reducing the meridional
overturning circulation. Data from cores through the Greenland ice sheet
(1), deep-sea sediments (2,3), and alpine-lake sediments (4) indicate
that: Left: During
recent times the circulation has been stable, and the polar front which
separates warm and cold water masses has allowed warm water to penetrate
beyond Norway. Center: During
the last ice age, periodic surges of icebergs reduced salinity and reduced
the meridional overturning circulation, causing the polar front to move
southward and keeping warm water south of Spain. Right: Similar
fluctuations during the last interglacial appear to have caused rapid,
large changes in climate. The Bottom plot is a rough indication of temperature
in the region, but the scales are not the same. From Zahn (1994).

The observed changes can be explained by the shut-down of the meridional
overturning circulation.

Melting icebergs lower the salinity of the surface water of the
North Atlantic (icebergs are made of frozen fresh water).

The fresh water never becomes dense enough to sink. Less salty
water is less dense that the cold salty water deep in the North
Atlantic.

If the water doesn't sink, warm water is not drawn to the far
north Atlantic.Top:The
Atlantic overturning circulation carries a tremendous amount of
heat northward, warming the North Atlantic region. It also generates
a huge volume of cold, salty water called North Atlantic Deep Water—a
great mass of water that flows southward, filling up the deep Atlantic
Ocean basin and eventually spreading into the deep Indian and Pacific
Oceans. Bottom: Paleoceanographers
have found evidence for very different patterns of ocean circulation
in the past. About 20,000 years ago (bottom), waters in the North
Atlantic sank only to intermediate depths and spread to a far lesser
extent. When that occurred, the climate in the North Atlantic region
was generally cold and more variable.
From The Once
and Future Circulation of the Ocean. (Illustration by E. Paul Oberlander,
Woods Hole Oceanographic Institution)

The surface circulation of the North Atlantic, including the
Gulf Stream, turns eastward, flows toward the Azores, and then
southward. The polar front is at the northern edge of this circulation.

Water north of the polar front freezes. No heat is released from
the ocean to the atmosphere to warm Europe, and the northern hemisphere
cools.

After many hundreds of years, the water in the North Atlantic
becomes salty and begins to sink, pulling warm water northward,
and the cold period ends.

The circulation cannot turn on as soon as the surface water becomes
salty. It must become saltier than normal. Thus once the circulation
shuts off, it remains off for centuries. The delay is due to a
process called hysteresis.

The meridional-overturning circulation is
part of a non-linear system. The circulation has two stable states
near 2 and 4.
The switching of north Atlantic from a warm, salty regime to
a cold, fresh regime and back has hysteresis. This means that
as surface water in the north Atlantic (1 in the figure) becomes
less salty (2 in the figure) it
quickly switches to a cold, fresh state (3
in the figure).
For the system to switch back to the original state 1, the
surface water must become much more salty than at 3 (4 in the figure). This
type of behavior is called a hysteresis loop.

The controversy now is: Will this happen again?

Will ice melting in Greenland and the Arctic, due to recent global
warming, produce enough fresh water to reduce or shut down
the meridional circulation?

Some ocean-atmosphere climate models suggest it is possible,
but not likely.

Calculated changes in the Atlantic meridional overturning circulation
[strength of the circulation given in Sverdrups (Sv); 1 Sv = 106
m3/s] for a simplified, coupled, ocean-atmosphere, numerical
model for 100 model realizations. Radiative
forcing is increased from years 1000 to 1140, equivalent to a doubling
of CO2, and then held constant. The warming pushes the
model closer to the bifurcation point, and transitions usually occur
when the overturning is weakened. Two individual realizations are
highlighted by the black lines, one in which the THC remains strong
but highly variable, and one in which the THC undergoes a rapid transition
much later than, and completely unrelated in time to, the forcing.
Transitions occur preferentially following a notable reduction of
the meridional overturning circulation.
From Alley et al. (2003).

If the circulation shuts down, climate will change. The northern
hemisphere is likely to get much colder, but not as fast as in the movie Day
After Tomorrow.

Feedback

Abrupt climate change is due to positive (self-reinforcing)
feedbacks that push the climate system into a new, stable state. Other
processes inhibit change and stabilize climate, they are negative (self-limiting)
feedbacks. Thus, knowledge of feedback is important for understanding
abrupt climate change.

Over the past 10,000 years, earth's climate has been remarkably
constant. This requires negative feedbacks that stabilize climate. Over
other time periods, external forcing such as the amount of sunlight reaching
high northern latitudes, pushes the climate system out of equilibrium
to the point where positive feedbacks dominate. The positive feedbacks
quickly change climate until negative feedbacks again dominate, and
the climate system settles into a new, but different state. The forcing
that pushes the climaye system out of equilibrium can be external, such
as changes in insolation or volcanic eruptions, or internal, such as
natural changes within the climate system. El
Niño is an example of change due to internal forcing.

Here is an animation of two possible states of a system.
Initially, external forcing shown by red arrows is too weak to force
the ball (which represents the climate system) out of the original state.
The ball stays in the valley on the left because negative feedback due
to gravity forces the ball back to the bottom of the valley. Eventually,
the external forcing is great enough to force the ball to the pass between
the two valleys. At the pass, the ball could be stable. But if it moves
slightly to the right or the left, positive feedback takes over and rapidly
pushes the ball back to the original valley, or rapidly into the new,
higher valley on the right. In this example, the force of gravity causing
the ball to move is proportional to the slope of the ground (green surface).
In the valleys and at the pass, the slope is zero (level ground) and
gravity does not move the ball. On sloping ground, the gravity causes
the ball to move downhill.

Feedback must increase or decrease the rate of change the original
variable. If global surface temperature is the original variable, feedback
must either amplify or reduce change in global surface temperature.

If the process leading back to the original variable does not change
the original variable, the process is a cycle, such as the carbon cycle.
It is not a feedback. In a very simple carbon cycle, phytoplankton
grow, absorbing carbon dioxide from the ocean. The phytoplankton then
die, decay, and release carbon dioxide back to the ocean. This is a
cycle because it does not change the concentration of carbon dioxide
in the ocean.

Positive feedback cannot continue indefinitely. It eventually pushes
the system into a new state dominated by negative feedback which reduces
the rate of change.

Examples of feedback loops include:

Positive Feedback (self-reinforcing or amplifiers): More water vapor
in the atmosphere increases the greenhouse effect, which warms the
ocean, which leads to more evaporation, which leads to more water vapor
in the atmosphere. This is a positive feedback. The original variable
is water vapor, and the loop increases the rate at which water vapor
in the atmosphere changes. It increases faster and faster.

Negative Feedback (self-limiting or source of persistence): More
water vapor in the atmosphere leads to more clouds. Clouds reflect
incoming solar radiation (energy), leading to cooling of earth's surface,
which reduces evaporation. This is a negative feedback. Again, water
vapor is the original variable, but this loop reduces the change in
the variable, leading to stability (persistence).

Much work is now being done to try to understand which feedbacks dominate
the climate system.

An example of a system that has two stable states (top), with negative
and positive feedback.
Small displacements of the system from stable state 1 (middle) have
negative feedback pushing the system back to state 1.
Large displacements (bottom) have positive feedback pushing the system
to state 2.
From The
Argentine Foundation for Scientific Ecology.

The climate system has many feedback loops, including many that are
positive (causing amplified change). The climate system is non-linear.
Such systems can have abrupt changes when forcing becomes sufficiently
strong. For the earth's climate system, the forcing is coming from changes
CO2 in
the atmosphere. If enough CO2 is put into the air, the system
may change in ways we cannot anticipate. The changes in the meridional
overturning circulation is one example of abrupt change. But others are
possible. Many smaller changes have already been observed. They provide
warning that other, larger changes are possible. Do we want to wait for
the "violent
response"?

Additional Reading

For more on changes in the north Atlantic, read
the Great
Climate Flip-Flop, originally written for the Atlantic
Monthly, by William
H. Calvin which describes what we know about the role of the ocean
in the last ice age, and implications for the future.

For more on abrupt climate change, read Abrupt
Climate Change, a 2003 article in Science by a team
of oceanographers and climatologists. Other, more scientific, articles
are available in the Proceedings of the National Academy of Science
Volume 97, Issue
4.

[If you are a Texas A&M University student, and if you have difficulty
downloading the paper, you will
need to go through the library
site for e-resources. Click the radio button for E-Journal, then
type in the name of the journal, Science in this case.

Steffensen, J. P., K. K. Andersen, et al. (2008). High-Resolution
Greenland Ice Core Data Show Abrupt Climate Change Happens in Few Years.
Science 321 (5889): 680–684.
The last two abrupt warmings at the onset of our present warm interglacial period,
interrupted by the Younger Dryas cooling event, were investigated at high temporal
resolution from the North Greenland Ice Core Project ice core. The deuterium
excess, a proxy of Greenland precipitation moisture source, switched mode within
1 to 3 years over these transitions and initiated a more gradual change (over
50 years) of the Greenland air temperature, as recorded by stable water isotopes.
The onsets of both abrupt Greenland warmings were slightly preceded by decreasing
Greenland dust deposition, reflecting the wetting of Asian deserts. A northern
shift of the Intertropical Convergence Zone could be the trigger of these abrupt
shifts of Northern Hemisphere atmospheric circulation, resulting in changes of
2 to 4 kelvin in Greenland moisture source temperature from one year to the next.